310 I). II WENR1CH 



intensity of the light. Nevertheless, whenever the card was 

 moved upward the animals gave immediate and vigorous re- 

 sponses by closing the valves or by contraction movements of 

 the vela and tentacles. The reactions followed alike when the 

 movement of the card was slow — i.e., about 5 centimeters per 

 second — or when the card was suddenly jerked upward. Similar 

 reactions were also obtained when the card was moved down- 

 ward, or from side to side at a constant level. When a black 

 card was substituted for a white one, reactions sometimes re- 

 sulted, sometimes not. I cannot be certain whether there was 

 enough contrast in the amount of light reflected from the back- 

 ground, compared to that from the card, to allow the animals 

 to form an image of the moving black card, or whether some 

 slight movement of the arm or body of the observer in connec- 

 tion with the puling of the thread may not have been the source 

 of the stimulus. That the eyes were concerned in the recep- 

 tion of the stimuli from the moving cards is indicated by the 

 results of the following experiment. In all the previous experi- 

 ments the animals were always placed with the open, or ven- 

 tral, side toward the moving card. Now they were turned around 

 180 degrees so that their dorsal, or hinge, side was toward the 

 card. In this position light from the card could not fall on the 

 eyes and the animals gave no responses to the movement. Next 

 they were turned back 90 degrees, so that either the anterior 

 or posterior margins with their eyes were turned toward the 

 card. In this position they gave fully as vigorous responses as 

 when the ventral side was toward the card. 



In one series of experiments the size of the white card was 

 successively reduced to determine the lower limit of size to which 

 the animals would respond. The smallest card, the movement 

 of which produced a definite reaction, was 1.5 centimeters square 

 and its d' stance from the animal about 35 centimeters. No 

 attempt was made to determine a larger limit, nor any dis- 

 tance limits. 



It is believed by the writer that these experiments with the 

 upward moving white card on a black background, since any 

 possibility of the movement being accompanied by a diminu- 

 tion of the intensity of the light was excluded, and since definite 

 and vigorous responses were obtained, constitute a demonstra- 

 tion of the ability of the Pecten eye to form an image. Perhaps 



